Field of the Invention
The present invention relates to an image formation device configured such that a charge voltage formed by superimposition of an AC voltage on a DC voltage is supplied to a charging member to charge an image carrier.
Description of the Related Art
Examples of the technique of charging an image carrier such as a photosensitive drum in an image formation device such as a printer include the technique of charging an image carrier by a charging member, such as a charging roller and a charging brush, disposed in contact with a surface of the image carrier or disposed close to the surface of the image carrier with a certain spacing. In this charging technique, it is often configured such that a charge voltage formed by superimposition of an AC voltage on a DC voltage is supplied to the charging member.
JP 2001-201920 A discloses a configuration in which the level of peak-to-peak voltage is set to a proper value to stably perform discharging between an image carrier and a charging member based on the premise that there is the effect of averaging charge of the image carrier when a peak-to-peak voltage of an AC voltage has a value of equal to or greater than twice as great as a charge start voltage.
Specifically, in each of a first range in which the peak-to-peak voltage is lower than twice as high as the charge start voltage and a second range in which the peak-to-peak voltage is equal to or higher than twice as high as the charge start voltage, AC voltages with different detection peak-to-peak voltages are sequentially applied to the charging member, and the value of alternating current flowing through the charging member is sequentially detected.
Based on each detection value of the alternating current flowing through the charging member, an approximate function fI1(Vpp) of the alternating current value for the peak-to-peak voltage in the first range and an approximate function fI2(Vpp) of the alternating current value for the peak-to-peak voltage in the second range are obtained. Then, a peak-to-peak voltage value when a difference [=fI2(Vpp)−fI1(Vpp)] between the approximate functions fI1(Vpp), fI2(Vpp) is a predetermined value D is determined as a proper value.
However, as a result of experiment by the inventor(s) of the present invention, it has been found that the proper peak-to-peak voltage value is not always obtained in the configuration in which the above-described difference is fixed to the predetermined value D.
Specifically, even when the peak-to-peak voltage value obtained from the predetermined value D is proper in a brand-new state of the image carrier, if an image carrier surface is progressively worn out due to repeated printing for a long period of time, the peak-to-peak voltage value obtained from the same predetermined value D becomes extremely greater than a proper value at each point due to, e.g., a decrease in an electric resistance value of the image carrier. This leads to great damage on the image carrier. As a result, wearing out of the image carrier is further accelerated, and the image carrier early reaches the end of the life thereof.